The present invention relates to a particular kind of pick-up plugs, i.e., devices which are adapted to engage respective plastic preforms in view of supporting, orienting and carrying the preforms throughout the various steps which the preforms themselves go through for further processing, and in particular, during the phases of temperature conditioning and blow molding.
The term “pickup plug” is a term taken from the technical parlance that is commonly used, where other terms such as “tournette”, “mandrel”, “preform holder”, “carrier” are also used to identify the same device.
Therefore, the use of the term “pick-up plug” in this context should not create any difficulty or confusion to those skilled in the art.
The task that a pick-up plug is required to perform lies in engaging a respective preform by being inserted therein, and then supporting the preform, carrying the preform through various processing steps, and finally releasing the preform at the end of a processing cycle, by slipping off of the preform.
When chains of pick-up plugs are used in blow-molding installations, the number of such pick-up plugs is generally very large and this imposes, for obvious reasons of costs and reliability, as simple a construction as possible.
Pick-up plugs can be subdivided into two main categories, i.e., inner-engagement devices and outer-engagement devices.
Pick-up plugs for inner engagement work by being inserted in the neck portion of the preform, whereas pick-up plugs for outer engagement work by letting the neck portion of the preform into a portion thereof.
Generally speaking, the use of such pick-up plugs is connected with the following problems and drawbacks:
the neck portions of the individual portions are actually variable in their size and quite often exceed the specified dimensional tolerance limits;
an insertion of pick-up plugs in preforms that are too small, out of tolerance or at the limit thereof, proves quite difficult, if not almost impossible to be carried out, since this would require a lot of strain with the risk of the preform breaking down; and
preforms that are too large, out of tolerance or at the limit thereof, fall off during their transport.
These problems and drawnbacks create real risks of shutdowns, and related downtimes, of the entire production plant, with serious and easily imaginable economic consequences.
Pick-up plugs are known in the art which are provided with an annular spring that is intended to ensure a firm grip of the preforms by slipping thereinto, wherein such springs may be formed in a toroidal or a cylindrical shape.
These pick-up plugs, however, have the following drawbacks:
the gripping force, i.e., the effect exerted by the spring depends to a considerable extent on the actual diameter of the preform; in fact, it may quite easily occur that such effect changes all too quickly from a condition of an inadequate or even non-existing grip in the case of a perform that is too large, to a condition of an excessively strong grip which results in a stoppage due to jamming, in the case of a preform that is too small; and
in the case of such a jamming or stoppage situation, considerable axial insertion or disengagement thrust pushes the annular spring against the edges of the related housing, thereby increasing the force of friction opposing the contraction thereof (which would in fact enable the preform to move).
A penalty could furthermore be imposed by an effect deriving from the arc of the annular spring being squeezed, as is illustrated schematically in FIG. 1.
Devices based on the use of such annular springs, owing to the inherent nature thereof, do not allow for their axis to be maintained in position in an adequately accurate manner (the ring tends to move and get displaced in its housing) and, during the passage through conditioning ovens, the preforms rotate about an off-centre axis; and this of course causes the preforms to undergo a non-symmetric heating effect.
All of the above cited drawbacks refer to pick-up plugs for inner engagement of the preforms. In the case of pick-up plugs intended for outer engagement of the preforms, similar drawbacks are experienced, which should however be described in a reverse manner, owing to the different and symmetrical coupling intervening between an outer-engagement pick-up plug and the preform. However, since such a description is perfectly and readily imaginable by all those skilled in the art, it is intentionally omitted here for reasons of greater brevity.
Inner-engagement pick-up plugs that make use of rings made of elastic polymeric material are also well known in the art; and although these pick-up plugs actually seem effective in reducing or doing away with some of the above cited drawbacks, they still have a number of other drawbacks that may be summarized as follows:
difficulty in obtaining a stable, constant quality, as well as adjus bent and sizing difficulties;
sensitiveness of the polymeric material to the heat developed in conditioning ovens, resulting in a quick deterioration of the quality and the performance 10 capabilities thereof; and
a need for an additional mechanical function to be provided to ensure compression and decompression of the annular elastomeric joint.
Also known in the art, e.g., from the disclosure in U.S. Pat. No. 4,678,425 in the name of Cincinnati Milacron Inc., there are pick-up plugs in which the function of engaging the respective preform is assigned to a cylindrical elastic portion of the pick-up plug to be inserted in the preform, as well as to an appropriate annular protuberance (bulge 70) provided on the elastic portion. However, this solution has some serious drawbacks, such as in the first place the utmost rigidity (in the sense of aptitude to comply) with respect to even very small variations in the inside diameter of the neck portion of the preform.
In conclusion, currently employed pick-up plugs prove quite difficult to adjust and scarcely adaptable to the dimensional variations of the preforms; and as a result, they give rise to reductions in the industrial efficiencies of the production plants using them, owing to frequent accidents occurring in the insertion or release of the preforms, or even during the transport thereof.
Moreover, the need arises for all pick-up plugs in a plant to be duly replaced whenever the type of preforms being processed is going to be changed, regardless of such preforms being more or less dimensionally similar to the previously processed ones.